A number of prior art devices exist which may be used to stabilize or fuse bones. One major group of these devices is known in the art as bone screws. Typically, bone screws are used to stabilize fractures of small bones in the body, such as those found in the hands or feet. Once implanted, these screws are completely contained within the bone which is particularly necessary in those circumstances where a protruding bone screw would otherwise damage a nearby joint.
Representative examples of a common bone screw include those disclosed in U.S. Pat. No. 4,175,555 to Herbert. The bone screw of this reference is known as a "Herbert" screw and is characterized by a shaft with screw threads on the leading and trailing ends thereof. The threads are like handed, but of different pitch. In the case of a fracture where it is necessary to join a remote bone fragment back to the major bone structure, the thread pitch of the leading end slightly exceeds that of the trailing end which causes the bones to be brought together in compression as the bone screw is screwed into place. Compression is desirable both for fracture healing and stability. Conversely, in some situations, it may be necessary to apply distraction between two bone fragments. Accordingly, the pitch of the trailing end thread may be made greater than that of the leading end. A cannula or lumen may be formed through the shaft to accommodate a guide wire to guide the bone screw to the proper location.
Another example of a bone screw is U.S. Pat. No. 5,019,079 to Ross. Like the Herbert screw, the purpose of this bone screw is also for stabilization of a fracture by providing two sets of threads which place the bones in compression. A modified intermediate or middle section is disclosed which is explained as providing better lateral stability for the joined bone structures.
Another group of devices generally known as fusion cages are used to achieve fusion between two adjacent vertebral bodies. The purpose and functioning of these fusion cages are quite different from those of bone screws. One example of a prior art spinal fusion cage is U.S. Pat. No. 5,015,247 to Michaelson. This device is characterized by a generally cylindrical or tubular member having a series of macro-sized openings which communicate with a hollow core, and a series of uniform threads extending the length of the cage. In order to achieve fusion between two vertebrae, the fusion cage must be sized so that it is wider than the disc which normally spans between the adjacent vertebral bodies, and which has an area for contacting the adjacent vertebral bodies. The threads on the cage are not for purposes of compression or distraction between two vertebrae, but rather are used for securing the cage at diametrically opposed sides with the adjacent vertebrae. The cage may be filled with bone graft and the macro-sized openings provide pathways by which bony ingrowth may occur. Eventually, a patient's body incorporates the bone graft and the fusion cage becomes structurally united with the joined vertebrae. The threads used on fusion cages must be uniform throughout the length of the cage so that the cage is advanced through the adjacent vertebrae uniformly. Unlike a bone screw, if threads of differing pitches were used on the fusion cage, the trailing set of threads would destroy the bone grooves previously filled by the forward advancing threads. Accordingly, the surrounding bone would be destroyed and the threads would actually detract from securing the adjacent vertebrae.
Another example of a fusion cage is disclosed in U.S. Pat. No. 5,766,253 to Brosnahan, III. As with the Michaelson patent, this patent also discloses a generally cylindrical cage with uniform threads extending the length of the device. In order to achieve bony ingrowth for this cage, two indentations are formed on the outer surface of the cage and the indentations are filled with bone graft. The cage is then positioned in contact with end plates of the adjacent vertebrae.
As can be seen from a review of these prior art references, a bone screw spans transversely across a gap between two bone structures, such as caused by a bone fracture, and the bone screw is used to compress or distract the two bones. A fusion cage, on the other hand, extends longitudinally along the gap between two vertebral bodies which is normally filled by a disc, and the cage is used like a spacer to replace the disc. Therefore, these devices are used for quite different purposes and are implemented at 90.degree. from one another with respect to a gap between two opposing bone structures.
Until recently, many complaints of lower back pain and leg pain have been attributed to herniated discs or other injuries to the spinal column. Extensive therapy and treatment has often been unsuccessful in alleviating such pain. It has been recently found that some of this lower back and leg pain can be attributed to symptomatic sacroiliac dysfunction or instability. Normally, the sacroiliac joint which spans between the sacrum bone and ilium bone has nutation of one to two degrees. "Nutation" is the medical term which describes the relative movement between the sacrum and ilium. A patient's sacroiliac joint can become damaged resulting in hypermobility of the joint. Because of the small range of motion in the sacroiliac joint, hypermobility is very difficult to diagnose. Therefore, lower back pain or leg pain caused by sacroiliac dysfunction often goes misdiagnosed or undiagnosed.
Accordingly, it is one general objective of this invention to provide a device for correcting symptomatic sacroiliac dysfunction or instability. It is another general object of this invention to provide a device which enhances stability and compression for purposes of immobilizing a joint, and for fusing two opposed bone structures across the joint.